Determination of carbon in fly ash from microwave attenuation and phase shift

ABSTRACT

In the determination of the unburnt carbon content of fly ash, a microwave signal is transmitted through or reflected from a fly ash sample and the attenuation and phase shift of the transmitted or reflected signal from receiver antenna or circulator respectively, is determined with respect to the incident signal and used to provide a measure of unburnt carbon content. The invention is applicable to measurement of fly ash samples taken from a boiler outlet duct or for direct measurement of the unburnt carbon content of fly ash entrained in flue gas.

TECHNICAL FIELD

This invention relates to the measurement of the unburnt carbon contentof fly ash produced by a coal fired boiler.

BACKGROUND ART

In the combustion of pulverised coal for steam generation in coal-firedpower stations there are certain fixed losses determined for example, byplant design, and certain controllable losses caused by operating undernon-ideal conditions. The controllable losses comprise:

(a) losses due to incomplete combustion of both solids and combustiblegases;

(b) losses due to the need for excess air.

In practice the controllable losses show a minimum as a function ofoxygen in the flue gas and it is preferable to operate near thisminimum. One way this can be achieved is by basing control of the boileron the measurement of oxygen and carbon monoxide in flue gas. Most largeboilers today are equipped with oxygen analysers which measure O₂ at onepoint in a duct. A problem with these analysers is that the reading isdrastically distorted by air infiltration into the furnace and in theconvection passages downstream of the burners. Also, as measurements aremade at one point, sampling errors are large.

Carbon monoxide in flue gas stays at very low levels at high excess airand rises as excess air is reduced. Infrared CO analysers are availablewhich direct the IR beam across the stack, thus minimising samplingerrors. However, optimising excess air using CO monitors generallyproduces a large amount of unburnt carbon in the ash, because CO levelsare very low at optimum excess air.

An alternative technique is to base control of the boiler on thedetermination of unburnt carbon in the fly ash. A 500 MW power stationburning black coal of 20% ash will produce about 2500 tonnes/hr fluegas, and 37 tonnes/hr fly ash. The carbon content of this fly ash willbe normally in the range 2-5 wt % although it may contain up to 15 wt %carbon. Typically the fly ash concentration in flue gas is about 20g/m³. Present instruments for the determination of the carbon content ofthe fly ash rely on extracting a sample, typically less than 1 gram,from the duct and analysing this on a batch basis typically at 10-20minute intervals.

One prior art carbon concentration monitor [Rupprecht and PatashnickCo., Inc, NYSERDA Report 86-2, Jan. 1986]is based on a microbalance andsmall furnace. The instrument collects a 10-50 mg sample of fly ash fromthe outlet duct of a boiler and determines the unburnt carbon in thissample from the mass loss after heating at 750° C., this measurementcycle being repeated at approximately 15 minute intervals. Onedisadvantage of this analysis technique is that it is very difficult tocollect a representative sample of such small size, and thereforesampling uncertainty significantly limits the accuracy of the unburntcarbon determination. The analysis accuracy for replicate samples inlaboratory tests was approximately ±0.5 wt % at 2.3 wt % carbon.

Another commercially available device [Energy and Environmental ResearchCorporation, 18 Mason, Irvine, CA, USA; Dec 1987]for the determinationof unburnt carbon in fly ash collects an approximately 1 gram samplefrom the duct using an isokinetic sampler and analyses this for unburntcarbon content from the measured surface reflectance of the sample. Thesample collection and measurement cycle is repeated at approximately 5minute intervals. In a plant test of the instrument at the Nefo powerplant, Denmark, the analysis accuracy was approximately ±1 wt % at lessthan 3 wt % carbon and ±0.5 wt % at greater than 3 wt % carbon. Theanalysis accuracy is limited by sampling uncertainty, due to the samplesize and measuring principle (i.e. surface reflectance) used, and thesensitivity of the reflectance measurement to coal type.

A device based on a measurement of the capacitance of a fly ash filledcapacitor has been proposed for the determination of carbon in fly ashin Australian Patent 562440. In this arrangement ash is taken from anash hopper using a screw conveyor, fed into a measuring chamber into theelectric field established by the electrodes of a capacitor and thechange in capacitance of the capacitor measured, and finally returned tothe ash hopper using a second screw conveyor. The bulk density of theash in the measuring chamber is assumed to be approximately constant,although compensation for variation in the bulk density is possibleusing a weighing device.

A microwave technique has been proposed for simultaneously reducing andmeasuring the carbon content in fly ash in U.S. Pat. No. 4,705,409. Inthis technique ash is taken from an ash hopper and passed through ametallic waveguide. Microwave radiation directed through the guide ispreferentially absorbed by the carbon in the fly ash, and theconcentration of carbon is determined from measuring the temperaturerise of a water wall surrounding the guide. Sufficient microwave poweris injected into the guide to burn the excess carbon in the ash andgenerate a reduced carbon product. One disadvantage of this technique isthat the heat conduction out of the guide, and the associatedtemperature rise in the water wall, is a function of not only the carboncontent of the ash but also the chemical characteristics, temperatureand heat conduction properties of the ash. These factors need to betaken into acount in the calibration and operation of the device.

Nuclear measurement of carbon in fly ash has also been investigated[Steward, R.F., ISA Transactions, (3), 1967, 200-207]. In this techniquecarbon concentration is correlated with counts of 4.43 MeV gamma raysproduced from carbon atoms by the inelastic scatter of neutrons. Usingthis technique in laboratory measurements on 10 kg fly ash samples theanalysis accuracy is repeated as ±0.5 wt % over the range 2-16 wt %carbon.

DISCLOSURE OF THE INVENTION

It is an object of this invention to provide a method and apparatus tomeasure the unburnt carbon content in fly ash.

Accordingly, in one aspect this invention consists in an apparatus tomeasure the unburnt carbon content of fly ash comprising means togenerate a microwave signal, transmitter means to launch said microwavesignal for transmission through a fly ash sample, receiver means toreceive a signal Passed through the sample and processing means todetermine the attenuation and phase shift of the signal passed throughthe sample with respect to the launched signal and to produce from saidattenuation and phase shift a measure of unburnt carbon content.

In a second aspect this invention consists in an apparatus to measurethe unburnt carbon content of fly ash comprising means to generate amicrowave signal, antennae means to launch a microwave signal into a flyash sample and to receive a reflected signal and processing means todetermine the attenuation and phase shift of the reflected signal withrespect to the launched signal and to produce from said attenuation andphase shift a measure of unburnt carbon content.

In a third aspect this invention consists in a method of measuring theunburnt carbon content of fly ash comprising the steps of launching amicrowave signal into a fly ash sample, receiving the transmittedsignal, determining the attenuation and phase shift of the receivedsignal with respect to the launched signal and producing a measure ofcarbon content from said attenuation and phase shift.

In a fourth aspect this invention consists in a method of measuring theunburnt carbon content of fly ash comprising the steps of launching amicrowave signal into a fly ash sample, receiving a component of thesignal reflected from the sample, determining the attenuation or phaseshift of the reflected signal with respect to the launched signal andproducing a measure of carbon content from said attenuation and phaseshift.

In one preferred form of the invention separate microwave transmittersand receivers are used. These are provided with suitable antennae, forexample, horns or microstrip radiators in an open system, andcapacitative or inductive probes in waveguides.

In another preferred form of the invention a single transceiver is usedfor transmitting and receiving. This arrangement is particularlyadvantageous where a reflected signal is measured but can also be usedwhere a signal transmitted through the sample is measured by utilising asuitable microwave reflector and effecting a double pass cf the sample.

The microwave signal can be generated using any suitable microwaveoscillator. Preferably the frequency of the microwave signal is in therange of from 1 to 20 GHz.

The methods and apparatus of this invention can be used to measureunburnt carbon content of collected fly ash samples or of a fly ashsample entrained in the flue gas from a coal fired boiler. Measurementof unburnt carbon in the fly ash entrained in flue gas is preferablyperformed by locating suitable microwave transmitting and receivingantennae in the flue gas duct and measuring over a suitable pathlengtheither across or along the duct.

It will be apparent that the method and apparatus of this invention haveseveral advantages over the prior art. The measurements according tothis invention are non-destructive and require no special samplepreparation. The measurement of phase shift and attenuation can becompleted almost instantaneously and therefore a continuous measurementof unburnt carbon content can be provided. Further, the method andapparatus of this invention are not limited by sample size and can beused with samples varying from a few grams to tens of kilograms. Theability to analyse large samples allows sampling uncertainty to bereduced and enables improved measurement accuracy. The method andapparatus are also applicable to both collected samples and in situmeasurement.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 is a schematic block diagram of an apparatus to measure unburntcarbon in fly ash according to a first embodiment of this invention;

FIG. 2/is a schematic diagram of the antennae and sample measurementchamber in FIG. 1 for measurement in free space;

FIG. 3 is a schematic diagram of the antennae and sample measurementchamber in FIG. 1 for measurement in a waveguide;

FIG. 4 is a schematic diagram of the antennae and sample measurementchamber in FIG. 1 for measurement in a microwave/resonant cavity;

FIG. 5 is a graph showing correlation of (A/W) with wt % carbon formeasurement in a waveguide;

FIG. 6 is a graph showing correlation of (φ/W) with wt % carbon formeasurement in a waveguide;

FIG. 7 is a graph showing correlation of (A/W) with wt % carbon formeasurement in free space;

FIG. 8 is a graph showing correlation of (φ/W) with wt % carbon formeasurement in free space; and

FIG. 9 is a schematic diagram of an apparatus according to FIG. 1arranged for measurement of unburnt carbon in fly ash entrained in fluegas.

MODES FOR CARRYING OUT THE INVENTION

The propagation of an electromagnetic wave (EM) in a dielectric mediumis described by Maxwell's equations, and the complex amplitude given by

    E(1)=E.sub.o exp (-γ1)                               (1)

where 1 is the distance travelled by the EM wave in the dielectricmedium from some reference point where its amplitude was E_(o), and γ isthe propagation constant of the wave given by

    γ=α+jβ                                    (2)

where α and β are the attenuation and phase constants respectively. Fora non-magnetic dielectric medium α and β are given by ##EQU1## whereε_(o) is permittivity of free space, γ_(o) the wavelength is free space,ε' the dielectric constant of the medium and ε" the loss factor of themedium. The attenuation constant α represents the attenuation of the EMwave (in nepers per metre) and the phase constant β represents the phaseshift of the EM wave (in radians per metre).

From equations (3) and (4), it can be seen that the attenuation andphase shift of an EM wave in a dielectric is a function of the complexpermittivity of the medium,

    ε*=ε'-jε"                          (5)

For a multicomponent dielectric medium the complex permittivity may beapproximated by ##EQU2## where v_(i) and ε*_(i) are the volume fractionand complex permittivity of the i^(th) component respectively.

When a plane EM wave is incident upon a dielectric interface, part of itis reflected and part transmitted. For a non-magnetic dielectric in airthe reflection coefficient, R, and transmission coefficient, T, aregiven by ##EQU3## where E_(o), E_(R) and E_(T) are the incident,reflected and transmitted electric field vectors. From equations (3) and(4) it can be seen that the phase shift and attenuation of a transmittedmicrowave signal are functions of the effective complex permittivity ofthe sample given by equation (6). For fly ash the complex permittivityof the unburnt carbon is significantly different from the remainingmatrix which principally comprises oxides of silicon, aluminium andiron. Therefore the measured attenuation and phase shift for fly ash arestrong functions of the unburnt carbon content.

It will be seen from equation (7) that the reflection coefficient of amicrowave signal directed at a fly ash sample is also a function of thecomplex permittivity of the sample and the attenuation and phase shiftof a reflected signal are therefore also functions of the unburnt carboncontent of the samples.

In the method for determining unburnt carbon content of fly ashaccording to one aspect of this invention a microwave signal is directedthrough a fly ash sample using suitable transmitting and receivingantennae and the attenuation and phase shift of the signal due to thefly ash sample are measured. These are normally calculated as thedifference between the attenuation and phase shift determined with thesample and air. To compensate for variation in the density and thicknessof the fly ash sample the phase shift and attenuation can be normalisedto a unit sample mass per unit area. This is not necessary where thevariation in sample density and thickness can be maintained withinacceptable limits by a suitable sample presentation system.

To obtain a measure of unburnt carbon content in terms of weight percent(wt %) the attenuation or phase shift data are correlated with wt %unburnt carbon, determined by standard laboratory analysis, using leastsquares regression and equations of the form:

    wt % unburnt carbon =a.sub.O +a.sub.l (φ.sub.c)        (9)

    wt % unburnt carbon =b.sub.O +b.sub.l (a.sub.c)            (10)

where φ_(c) and A_(c) are the corrected (compensated for variation insample density and thickness) phase shift and attenuation respectively,and a₀,. . . b₁ are fitting constants. The unburnt carbon content mayalso be determined from a combined measurement of attenuation and phaseshift, independent of variation in sample density and thickness, usingan equation of the form

    wt % unburnt carbon =C.sub.0 +C.sub.1 (φ.sub.m)+C.sub.2 (A.sub.m)(11)

where φ_(m) and A_(m) are the measured phase shift and attenuationrespectively, and C₀,. . . ,C₂ are fitting constants.

In the method for determining unburnt carbon content of fly ashaccording to another aspect of the invention a microwave signal isdirected at a fly ash sample and the reflected signal detected. Either atransceiver or separate transmitting and receiving antennae can be usedfor transmitting and receiving the microwave signal. As with thetransmission method the attenuation and phase shift of the reflectedsignal are measured and preferably are correlated with wt % unburntcarbon using least squares regression and equations of the same form as(9), (10) and (11).

FIG. 1 schematically shows the arrangement of the apparatus to measureunburnt carbon content of fly ash according to this invention. As shownthe apparatus comprises a microwave source which takes the form of aYttrium-Iron-Garnet oscillator 1 tuneable over the range 2 to 4 GHz andcontrolled by a data logging computer 2. The output of oscillator 1 ismodulated by a PIN diode modulator 3 and directed through a low passfilter 4 to a power divider 5. Power divider diverts a small amount ofthe microwave signal to an 8-port junction 6 as a reference signal. Theremainder of the microwave signal is directed via a circulator 7 to atransmitter antenna 8. Circulator 7 is provided to direct any reflectedsignal to an appropriate instrumentation amplifier 9 to provide ameasurement signal for computer 2. Transmitter antenna 8 directs themicrowave signal through a sample measurement chamber 10 to a receiverantenna 11 from which the received signal is directed to 8-port junction6 and instrumentation amplifiers 9 to provide a measure of theattenuation and phaseshift of the received signal in the known manner.This data is transmitted for processing in the manner described herein.

The microwave antennae can be of any type suitable to the selectedsample presentation technique. FIGS. 2 to 4 show three preferredarrangements of the antennae and sample measurement chamber.

Referring to FIG. 2 an arrangement for measurement on an ash sample infree space. The antennae are horn antennae 12, 13 and the ash sample 14is contained in a container 15 formed of a material such as wood orplastic which allows the transmission of microwaves. In this arrangementthe ash sample 14 is packed in container 15 and suitably positionedbetween horns 12, 13. The phase shift and attenuation are determined asdescribed above and used to calculate the wt % of unburnt carbon asdescribed above.

FIG. 3 shows an arrangement for measurement on sample in a waveguide. Inthis arrangement the antennae are capacitive posts or inductive loops16, 17. The sample 14 to be measured is packed into a section ofwaveguide 18 of circular or rectangular cross section suited to thefrequency range of the microwave signal. For measurements in the 2.6 to3.95 GHz frequency range an RG-48 rectangular waveguide can be used. Thesample is confined to the selected region of the waveguide by plasticsheets 19 which allow transmission of the microwave signal. The phaseshift and attenuation are determined as described above and used tocalculate the wt % of unburnt carbon as described above.

FIG. 4 shows an arrangement for measurement on a sample in a microwaveresonant cavity. In this arrangement the ash sample is contained in aceramic tube 20 located along the axis of a TE mode resonant cavity 21.The microwave signal is coupled in and out of the resonant cavity usingH-field (inductive loop) probes 22, 23. The resonant frequency andQ-factor of the cavity are determined from a swept frequency measurementusing the 8-port junction shown in FIG. 1. The dielectric constant (ε')and loss factor (ε") of the ash sample are calculated from the measuredresonant frequency and Q-factor and used in Equations (3) and (4) todetermine the attenuation and phase constants, and the equivalentattenuation and phase shift. In addition, as ε' and ε" are generallydirectly proportional to phase shift and attenuation respectively, thesemay be substituted directly for these parameters in Equations (9)-(11).

The apparatus described with reference to FIGS. 1 and 2 and FIGS. 1 and3 respectively were used to perform measurements on a range of fly ashsamples from New South Wales and Queensland power stations. The unburntcarbon content of these samples was determined by standard chemicalanalysis using LECO analyser and was in the range 0.5 to 13 wt %. Thesamples were packed in an open container to a depth of approximately 100mm and in a 200 mm length of RG-48 waveguide section respectively, andthe phase shift and attenuation of a 3.3 GHz microwave signaldetermined. The data were correlated with wt % carbon using theequations,

    wt % carbon =a.sub.o +a.sub.l (φ.sub.fly ash /w)       (12)

    wt % carbon =b.sub.o +b.sub.l (a.sub.fly ash /w)           (13)

where a_(o),. . . ,b_(l) are fitting constants, w is sample mass perunit area (in g cm⁻²) and φ_(fly) ash and A_(fly) ash are the Phaseshift (in degrees) and attenuation (in dB) of the fly ash samplerespectively.

R.m.s. errors from correlations on the data using equations (12) and(13) are given below in Table 1.

                  TABLE 1                                                         ______________________________________                                                             R.m.s. Error                                                                  Measure-  (wt % Carbon)                                  Power    Unburnt     ment      Equation                                                                             Equation                                Station  Carbon (Wt %)                                                                             Geometry  (12)   (13)                                    ______________________________________                                        Wallerawang                                                                             3-13       Free space                                                                              0.41   1.41                                                         Waveguide 0.28   1.22                                    Swanbank 0.5-5       Free space                                                                              0.17   0.83                                                         Waveguide 0.22   0.70                                    Eraring  0.5-2.5     Waveguide 0.19   0.29                                    ______________________________________                                    

Plots of the phase shift and attenuation data for Swanbank fly ashsamples are presented in FIGS. 5 and 6 for measurements in waveguide andFIGS. 7 and 8 for measurements in free space. The r.m.s. errors in Table1 represent the total analysis error due to gauge inaccuracy, samplingand chemical analysis. These results indicate that a measurement ofphase shift is the most accurate for the determination of carboncontent, and the accuracy of analysis is comparable to or better thanthat obtained with previous methods.

The apparatus described above is particularly suitable for on-lineanalysis of the unburnt carbon content of fly ash sampled from a boileroutlet duct. Fly ash is removed from the boiler outlet duct byconventional sampling means (not shown), for example using a Cegritsample and cyclone, and passed through the sample measurement chamber ofthe apparatus. The fly ash can be fed continuously or in batches, andcarried to and from the measurement chamber by any suitable means, forexample by a screw conveyor.

FIG. 9 shows the apparatus described with reference to FIGS. 1 and 2arranged for measurement of the unburnt carbon content of fly ashentrained in flue gas in a boiler outlet duct 24. As shown the microwavesignal is transmitted across the duct perpendicular to the gas flowdirection. Waveguide or resonant cavity arrangements of the kind shownin FIGS. 3 and 4 respectively can equally be utilised for measurement ofthe unburnt carbon content of fly ash entrained in flue gas of a boileroutlet duct.

The apparatus shown in FIG. 9 is particularly suitable for on-lineanalysis of the unburnt carbon content of fly ash entrained in flue gas.

The foregoing describes the invention with reference to some specificexamples and it will be apparent to those skilled in the art thatmodifications can be made without departing from the scope of theinvention.

I claim:
 1. An apparatus to measure the unburnt carbon content of flyash comprising means to generate a microwave signal, transmitter meansto launch said microwave signal for transmission through a fly ashsample, receiver means to receive a signal passed through the sample andprocessing means to determine the attenuation and phase shift of thesignal passed through the sample with respect to the launched signal andto produce a measure of unburnt carbon content.
 2. An apparatus asclaimed in claim 1 further comprising a measurement chamber to containsaid fly ash sample.
 3. An apparatus as claimed in claim 2 wherein saidtransmitter means and receiver means are horn antennae and saidmeasurement chamber is formed from a material permitting transmission ofmicrowaves.
 4. An apparatus as claimed in claim 2 wherein saidtransmitter means and said receiver means are capacitive post antennaeand said measurement chamber is a section of waveguide.
 5. An apparatusas claimed in claim 2 wherein said transmitter means and said receivermeans are inductive loop antennae and said measurement chamber is asection of waveguide.
 6. An apparatus as claimed in claim 2 wherein saidtransmitter means and receiver means are inductive loops and saidmeasurement chamber is disposed within TE mode microwave resonantcavity.
 7. An apparatus as claimed in claim 2 wherein said transmittermeans and receiver means are capactive post antennae and saidmeasurement chamber is disposed within a TE mode microwave resonantcavity.
 8. An apparatus as claimed in claim 6 or claim 7 wherein the flyash sample is disposed about the axis of the cavity.
 9. An apparatus tomeasure the unburnt carbon content of fly ash comprising means togenerate a microwave signal, antennae means to launch a microwave signalinto a fly ash sample and to receive a reflected signal and processingmeans to determine the attenuation and phase shift of the reflectedsignal with respect to the launched signal and to produce from saidattenuation and phase shift a measure of unburnt carbon content.
 10. Anapparatus as claimed in claim 9 wherein said microwave signal islaunched and received by a microwave transceiver.
 11. An apparatus asclaimed in claim 9 wherein said microwave is launched and received byseparate antennae.
 12. An apparatus as claimed in claim 9 furthercomprising a microwave reflector disposed on the distal side of said flyash sample to said antenna means to reflect a microwave signal passedthrough the fly ash sample back through the sample to said antennaemeans.
 13. A method of measuring the unburnt carbon content of fly ashcomprising the steps of launching a microwave signal into a fly ashsample, receiving a transmitted signal, determining the attenuation andphase shift of the received signal with respect to the launched signaland producing a measure of unburnt carbon content from said attenuationand phase shift.
 14. A method of measuring the unburnt carbon content offly ash comprising the steps of launching a microwave signal into a flyash sample, receiving a reflected microwave signal, determining theattenuation and phase shift of the reflected signal with respect to thelaunched signal and producing a measure of unburnt carbon content fromsaid attenuation and phase shift.
 15. A method as claimed in claim 14wherein said fly ash sample is entrained in flue gas of a boiler outletduct.
 16. A method as claimed in claim 14 or claim 15 wherein saidmicrowave signal is transmitted into said fly ash sample in free space.17. A method as claimed in claim 14 or claim 15 wherein said sample isdisposed in a waveguide.
 18. A method as claimed in claim 14 or claim 15wherein said sample is disposed in a TE mode resonant microwave cavity.19. A method as claimed in claim 14 further comprising the steps ofreflecting a microwave signal passed through said fly ash sample backthrough said fly ash sample and receiving that reflected signal whichhas passed through the sample twice.